The ultimate goal of this proposal is to identify the non-HLA platelet antigens that must be matched between donor and recipient to ensure platelet compatibility. This will be done by careful characterization of non-HLA platelet antibodies and antigens. Initially we will identify and categorize non-HLA platelet antisera. This will involve: 1) develop sensitive, reliable and comprehensive platelet antibody testing techniques; 2) evaluate platelet crossmatch tests to determine the transfusion relevant antibodies/angigens; 3) obtain antisera from appropriate sources (i.e., patients with post-transfusion purpura, mothers of infants with neonatal alloimmune thrombocytopenia, multiparous women and from planned platelet immunization programs); 4) test the sera with selected platelet panels to group the antisera into those detecting the same antigens; and 5) produce the relevant antisera in quantity using hybridoma technology. These well-characterized non-HLA platelet antisera will be used in family studies to detect the corresponding antigens. The same families will be tested for other genetic markers (HLA antigens, red cell antigens, red cell enzymes and plasma proteins) to determine the inheritance and genetic linkage and/or chromosomal assignment of non-HLA platelet antigens. Further analysis of non-HLA platelet antigens will be done by determining their expression on other body cells as well as their localization to specific glycoprotein fractions within the platelet membrane.